The present invention relates to a method of fabricating a medical component, such as a medical implant, from a biocompatible material having a relatively high concentration of a carbide or carbon source and to such medical component.
Medical components, such as medical implant components, may be formed or fabricated from a material or materials having good wear properties. As an example, such components may be formed or fabricated from a biocompatible material such as cobalt chrome or a cobalt chrome alloy having a carbide content. For medical implants, such carbide content may comprise a relatively small percentage of the final material, such as less than 6.17% by weight thereof and typically only approximately 3-5% by weight thereof.
The carbide content is primarily responsible for the good wear properties of the above-mentioned cobalt chrome alloy. As is to be appreciated, if the percentage of carbide content in a material (such a cobalt chrome alloy) could be increased, then the wear properties of the resultant alloy or material could be improved. However, increasing the carbide content may result in a decrease of other properties. For example, increasing the carbide content in a biocompatible material (such as cobalt chrome) may reduce the fatigue life, strength, corrosion resistance, and toughness, may produce a material which is relatively highly brittle, and/or may reduce the uniformity of the material and produce a material which is relatively highly non-uniform.
The decrease in the above-identified properties (especially the uniformity) may make the resultant material difficult to machine. More specifically, if the carbide content is increased beyond a certain amount, the carbide content in the biocompatible material may not completely mix with the biocompatible material. As a result, the biocompatible material may have some of the carbide constituent or particles completely mixed therein and may have some of the carbide particles which are not completely mixed or not at all mixed therein. Such situation may be considered similar to that of adding sugar to a glass of water. In this later situation, after a certain amount of sugar is added, the sugar no longer mixes or dissolves in the water. Instead, some of the sugar remains in a non-dissolved or a not completely dissolved state.
To further describe the above-mentioned machining difficulty of a material having an increased carbide content, consider the parts illustrated in FIGS. 3A and 3B. With reference to FIG. 3A, unmixed carbide particles 90 contained within an item 92 formed from biocompatible material and carbide may be relatively large, such as between 5-20 microns in size or length. Additionally, the carbide particles 90 may be relatively strong. As a result, machining or cutting such material properly may be difficult if not impossible. For example, and with reference to FIG. 3B, if a surface 94 of the item 92 to be machined contains a number of relatively large carbide particles 90, then during a machining operation thereof when a cutting tool 96 encounters a portion 98 of a respective carbide particle 90, instead of just the desired portion of such carbide particle being cut, the entire particle may be removed thereby leaving a depression in the surface. As such, it may be very difficult, if not impossible, to properly machine surface 94 (having the relatively large size carbide particles 90) to a desired thickness or dimension T. In other words, even if the item 92 is actually machined so as to have thickness/dimension T, the machined surface may contain a number of depressions or voids and, as such, may not have a desired surface roughness or finish. Additionally, since the carbide particles 90 are relatively strong, the cutting tool 96 may be damaged during the machining or cutting operation.
A description of a material which may be typically used for medical implant components will now be provided.
A material typically used in the fabrication of medical implant components is ASTM F75, ISO 5832, where CoCrMo alloy composing of 1-5 vol % carbides with atomic composition by weight percent of C 0.28-0.35, Cr 28.10-28.31, Mo 5.61-5.92, Si 0.95-0.96, Mn 0.36-0.40, Ni 0.27-0.73, Fe 0.14-0.24, W 0.04-0.05, Co balance, and other elements<0.001. The carbide phases are M23C6, M7C3, M3C2, and MC, where M is metallic elements of Cr, Mo, W. The primary phase is Cr23C6. Usually, as cast CoCrMo may have a carbide content of about 5% in volume. Merely increasing the carbide content in as-cast CoCrMo alloy may result in a decrease of corrosion resistance, strength, toughness, and fatigue life due to the inability of all of the carbide particles to go into solution and the tendency to precipitate at the grain boundary during solidification.
Additionally, another limitation associated with the use of the F75 CoCrMo alloy may be due to the large size of the carbide particles. As indicated by Cawley et al., the size of the carbide particles in F75 may be larger than 1.0 micron (1000 nm) and may be within the range of 10-100 μm. According to the Hall-Petch relationship, the hardness is inversely proportional to the square root of carbide size in alloys. In other words, the larger the size, the lower the hardness and, additionally the lower the strength and toughness.
Accordingly, it has been very difficult, if not impossible, to fabricate a medical implant or component from a biocompatible material having a relatively high carbide content, such as that of 6.17% by weight or higher.
It would be advantageous to provide a technique for fabricating a medical component, such as a medical implant, from a biocompatible material or alloy having a relatively high carbon or carbide content so as to increase the wear properties over that obtained from currently used biocompatible materials. It would be further advantageous to provide such technique whereby the biocompatible material or alloy would have relatively good fatigue properties, would not be highly brittle, and would be relatively uniform or homogeneous.